Refrigerant vapor compression systems are well known in the art. Air conditioners and heat pumps employing refrigerant vapor compression cycles are commonly used for cooling or cooling/heating air supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression systems are also commonly used for cooling air, or other secondary media such as water or glycol solution, to provide a refrigerated environment for food items and beverage products within display cases, bottle coolers or other similar equipment in supermarkets, convenience stores, groceries, cafeterias, restaurants and other food service establishments.
Conventionally, these refrigerant vapor compression systems include a compressor, a condenser, an expansion device, and an evaporator serially connected in refrigerant flow communication. The aforementioned basic refrigerant vapor compression system components are interconnected by refrigerant lines in a closed refrigerant circuit and arranged in accord with the vapor compression cycle employed. The expansion device, commonly an expansion valve or a fixed-bore metering device, such as an orifice or a capillary tube, is disposed in the refrigerant line at a location in the refrigerant circuit upstream, with respect to refrigerant flow, of the evaporator and downstream of the condenser. The expansion device operates to expand the liquid refrigerant passing through the refrigerant line, connecting the condenser to the evaporator, to a lower pressure and temperature. The refrigerant vapor compression system may be charged with any of a variety of refrigerants, including, for example, R-12, R-22, R-134a, R-404A, R-410A, R-407C, R717, R744 or other compressible fluid.
In some refrigerant vapor compression systems, the evaporator is a parallel tube heat exchanger having a plurality of flat, typically rectangular or oval in cross-section, multi-channel tubes extending longitudinally in parallel, spaced relationship between a first generally vertically extending header or manifold and a second generally vertically extending header or manifold, one of which serves as an inlet header/manifold. The inlet header receives the refrigerant flow from the refrigerant circuit and distributes this refrigerant flow amongst the plurality of parallel flow paths through the heat exchanger. The other header serves to collect the refrigerant flow as it leaves the respective flow paths and to direct the collected flow back to the refrigerant line to return to the compressor in a single pass heat exchanger, which in this case, serves as an outlet header/manifold, or to a downstream bank of parallel heat exchange tubes in a multi-pass heat exchanger. In the latter case, this header is an intermediate manifold or a manifold chamber and serves as an inlet header to the next downstream bank of parallel heat transfer tubes.
Each multi-channel tube generally has a plurality of flow channels extending longitudinally in parallel relationship the entire length of the tube, each channel providing a relatively small flow area refrigerant flow path. Thus, a heat exchanger with multi-channel tubes extending in parallel relationship between the inlet and outlet headers of the heat exchanger will have a relatively large number of small flow area refrigerant flow paths extending between the two headers. Sometimes, such multi-channel heat exchanger constructions are called microchannel of minichannel heat exchangers as well. Commonly, for evaporator applications, the heat exchanger generally includes heat transfer fins positioned between heat transfer tubes for heat transfer enhancement, structural rigidity and heat exchanger design compactness. The heat transfer tubes and fins are permanently attached to each other (as well as to the manifolds) during a furnace braze operation. The fins may have flat, wavy, corrugated or louvered design and typically form triangular, rectangular, offset or trapezoidal airflow passages.
When a heat exchanger is used as an evaporator in a refrigerant vapor compression system, moisture in the air flowing through the evaporator and over the external surfaces of the refrigerant conveying tubes and associated fins of the heat exchanger condenses out of the air and collects on the external surface of the those tubes and fins. In general, condensate naturally drains well from refrigerant vapor compression system evaporators having round heat transfer tubes and plate fins due to the cylindrical outer surface of a round tube and vertically extended plate fins. For evaporator heat exchangers having the flat tubes and serpentine fins arranged in a vertical orientation extending between a pair of horizontally disposed headers, such as, for example, the heat pump evaporator/condenser heat exchanger disclosed in U.S. Pat. No. 5,826,649, the condensate depositing on the heat transfer tubes and associated heat transfer fins inherently drains down the vertically extending tubes under the influence of gravity. The draining condensate is typically collected in a drain pan disposed beneath the heat exchanger.
U.S. Pat. No. 5,279,360 discloses an evaporator heat exchanger having an array of parallel heat exchange tubes of flattened cross-section disposed in spaced relationship with V-shaped fins disposed between the facing flat surfaces of adjacent heat exchange tubes. Each heat exchange tube is bent into a V-shape and disposed in a vertical plane with its inlet end connected in fluid communication with a first horizontally extending header and its outlet end connected in fluid communication with a second horizontally extending header. The apexes of the arrayed V-shape-bent heat exchange tubes are aligned at a lower elevation than the headers, and a condensate trough is disposed therebeneath. Condensate collecting on the flattened heat exchange tubes and the fins therebetween drains downwardly along the respective fin-free edge surfaces of the flattened heat exchange tubes to the condensate trough.
However, with respect to prior art heat exchangers having tubes of flattened cross-section disposed horizontally and extending longitudinally in a horizontal direction between a pair of spaced, generally vertical headers, condensate collecting on the upper side of the tubes does not drain therefrom because of the horizontal disposition of the flat external surface of the lube. If the condensate collecting on the external surfaces of the heat exchanger tubes becomes excessive, overall performance of the refrigerant vapor compression system will be adversely impacted. For example, excessive condensate retention on the external surfaces of the heat exchange tubes can result in increased air side pressure drop through the evaporator which causes increased fan power consumption and reduced heat transfer through the heat transfer tubes. Also, condensate collecting on the external surfaces of the heat transfer tubes of the evaporator may be undesirability re-entrained in the air passing through the evaporator and transversely over the flattened tubes. Further, under certain conditions, excessive condensate retention promotes faster frost accumulation and undesirably requires more frequent defrost cycles.